- Improve performance of plasmaURL.exists() by remembering URL-hashes that are not present
- Use a more realistic estimation of memory usage by the existsIndex cache
- Routine cleanup of the existsIndex to limit its memory usage
git-svn-id: https://svn.berlios.de/svnroot/repos/yacy/trunk@2113 6c8d7289-2bf4-0310-a012-ef5d649a1542